This invention generally relates to casting processes and materials. More particularly, the invention relates to materials and processes suitable for use in melting and casting operations, including the melting and casting of reactive materials.
While nickel-, cobalt- and iron-based superalloys have found wide use for components within gas turbine engines, especially for use in the hot gas path of these engines, alternative materials have been used and proposed to achieve various desired properties, including lower densities and higher temperature capabilities. Nonlimiting examples include reactive metals and their alloys (notable examples of which include niobium, titanium, zirconium, and their respective alloys), refractory metal intermetallic composite (RMIC) materials (notable examples of which include alloys based on niobium, titanium, hafnium and zirconium), and nickel-, cobalt-, and iron-base superalloys containing relatively high levels of reactive elements. In addition to the previously-noted reactive metals, other notable reactive elements include yttrium, cerium, cesium, tantalum, tungsten, rhenium and potentially other elements that tend to readily react with oxygen when molten or at an elevated temperature.
Components formed of reactive element-based materials are often formed by casting techniques, a notable example being investment casting (lost wax) processes. As known in the art, investment casting typically entails dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. Common materials for the refractory particulate material include alumina, silica, zircon and zirconia, and common materials for the binder include silica-based materials, for example, colloidal silica. A stucco coating of a coarser refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the shell mold, such as by heating, after which the mold is fired to sinter the refractory particulate materials and achieve a suitable strength. To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores are positioned within the shell mold to define the cooling channels and any other required internal features. Cores are typically made by baking or firing a plasticized ceramic mixture. One or more cores are then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the pattern. The pattern with its internal cores can then be used to form a shell mold as described above. Once the shell mold is completed and the pattern selectively removed to leave the shell mold and cores, molten metal is introduced into the shell mold and solidified to form the desired component, after which the mold and cores are removed.
Shell molds and cores used in investment casting processes must exhibit sufficient strength and integrity to survive the casting process. Additional challenges are encountered when attempting to form castings of reactive materials as a result of their high melting temperatures and reactivity, which have presented significant barriers to the use of conventional ceramic molds. For this reason, surfaces of molds and cores used in the casting of reactive materials are often provided with protective barriers known as facecoats. Facecoats are generally applied to mold and core surfaces in the form of a slurry, which may be dried and sintered prior to the casting operation or may undergo sintering during the casting operation. Typical facecoat slurries comprise a refractory particulate material in an aqueous-based inorganic binder, optionally with various additional constituents such as organic binders, surfactants, dispersants, pH adjusters, etc., to promote the processing, handling, and flow characteristics of the slurry. The refractory particulate material is chosen on the basis of being sufficiently unreactive or inert to the particular reactive material being cast. Various facecoat materials have been used and proposed, including those containing yttria (Y2O3), alumina (Al2O3), and zirconia (ZrO2) in a colloidal silica binder.
Yttria-containing facecoats have been particularly identified for use due to their relative inertness to reactive materials. However, a significant drawback is that conventional slurries suitable for producing yttria-containing facecoats exhibit a poor shelf life under typical mold room conditions. In particular, yttria-based slurries are prone to gelling, leading to poor application characteristics as well as casting surface defects. As a result, if not used within a relatively short time a yttria-based slurry must typically be discarded. Various solutions have been proposed to address the instability of yttria-based facecoat slurries, including control of the slurry pH (for example, above 10.2), fusing yttria with other oxides, and protecting the slurry from contact with air. While effective, less complicated and costly measures would be desirable.